Method for detecting eavesdroppers in a wireless communication system

ABSTRACT

The present invention relates to a method in a wireless communication system for determining whether an active eavesdropper is interfering in transmissions on a radio channel from a transmitter to a receiver, the method comprising the steps of: estimating, by said transmitter, a first effective channel gain of said radio channel at said receiver; estimating, by said receiver, a second effective channel gain of said radio channel at said receiver; and determining whether an active eavesdropper is interfering in the transmissions by comparing said first and second effective channel gains. Furthermore, the invention also relates to a method in a receiver, a receiver device, a computer program, and a computer program product thereof.

TECHNICAL FIELD

The present invention relates to a method in a wireless communicationsystem for determining whether an active eavesdropper is interfering ornot in transmissions between a transmitter and a receiver. Furthermore,the invention also relates to a method in a receiver, a receiver device,a computer program, and a computer program product thereof.

BACKGROUND OF THE INVENTION

In traditional crypto systems, each user is assigned a secret key, andthen e.g. a Base Station (BS) encrypts the messages to the mobile usersso that only the user with the correct key can decrypt and recover theoriginal message. This system can be made secure, but suffers from theproblem of key distribution. The distribution is both costly andunreliable. If there is an eavesdropper that somehow manages to stealthe key of some user, then the communication to that user is totallyun-protected. In the modern world, and in simulated wars, the mainstrategy to break a crypto system is to steal the key as there cannot beany “mathematical protection” against person B stealing person A's key.Further, distributing keys to, literally speaking, millions of customersare not easy tasks and impose significant economical overheads andcosts.

An alternative to key-based crypto systems is Physical Layer Security(PLS). In PLS, one exploits the fact that the user (the reader can viewa “user” as e.g. a mobile station or a UE in 3GPP LTE system) and theeavesdropper (an enemy that tries to overhear the transmission) havedifferent communication channels. Associated to each particular channelis its Shannon capacity. The Shannon capacity is the highest bit ratethat can be transmitted over the channel without any bit errors, and thereader can think of Shannon capacity as the maximal bit rate thathis/her mobile phone can operate with at any given time. This Shannoncapacity has absolutely nothing to do with crypto systems, but thefollowing classical results establishes a link.

Theorem of secrecy capacity (SC): Let C_(IU) be the Shannon capacity tothe Intended User (IU), and C_(ED) to the Eavesdropper (ED). Then,without any formal cryptosystem, the bit rate C_(SC)=C_(IU)−C_(ED) canbe transmitted with perfect security to the intended user.

Let us exemplify this result with a simple example: Person A is walkingaround downtown and is downloading documents at a bit rate of 10 Mbit/s,while the channel is actually so good that A could have downloaded at arate 25 Mbit/s (i.e. the Shannon capacity of the channel). Person A isintentionally backing off the peak rate. Now, another person B—theeavesdropper—is also downtown and can overhear the transmission toperson A, and the Shannon capacity of the channel to person B isslightly worse than to person A, namely 18 Mbit/s. According to thetheorem of SC, the SC is the difference of the two Shannon capacities,i.e. 25−18=7 Mbit/s. But, person A is downloading at a higher rate thatthe secrecy capacity, which implies that his transmission is not secureand person B can steal his data. If person A would have been careful,person A would have downloaded with a much smaller rate, say, 3 Mbit/s,and then person A's link would be safe even without any cryptosystem.

Now, the relation among the numbers in the previous example isrepresentative for “normal” (i.e. small) MIMO, systems but for massiveMIMO systems the situation changes drastically. Current multiple antenna(MIMO) systems use at most 8 antennas at the base station side. However,massive MIMO systems are actively researched and are one of the “hotareas” within the technical field. A massive MIMO system scales up thenumber of antennas with >1 order of magnitude, and a 1000 antenna basestation are not ruled out in these scenarios. Massive MIMO is likely tobecome a key technology in future 5G wireless systems.

For massive MIMO, the ratio of the Shannon capacity to the eavesdropperand the Shannon capacity to the intended user will be very close to 0;closer to 0 the more antennas at the base station. This implies that theSC is almost identical to the Shannon capacity to the intended user. Inour example, we would have that C_(IU)=25 Mbit/s, but a typical valuefor C_(ED) with 60-80 antennas would be, say, C_(ED)=0.5 Mbit/s. Hence,the user's data is perfectly safe as 10<25−0.5=24.5. In fact, the usercan download at rates very close the peak data rate and there is no needfor any crypto system as the link is guaranteed to be safe by thementioned Theorem of SC.

The inventor has therefore identified the neat result that a passiveeavesdropper (i.e., one that just walks around and listens to thechannel) cannot do any harm to the PLS of a massive MIMO system.Therefore, it has been concluded that a clever eavesdropper will changeinto an active mode by using a so called pilot attack. What theeavesdropper will do is simply to transmit some cleverly chosen signalswith the overall effect that some of the Shannon capacity to theintended user will be “stolen”, and the problem is that the intendeduser does not know that it is stolen, the intended user can only seethat the capacity is low.

Let us continue with our example. The eavesdropper transmits a few pilotsignals with the end effect is that C_(ED)=10 Mbit/s and C_(IU)=15Mbit/s, so that C_(SC)=5 Mbit/s, and since the data rate is 10 Mbit/s,which exceeds the SC, the transmission is no longer safe. The problem isthat the intended user only sees that intended user has C_(IU)=15Mbit/s, but has no idea that C_(ED)=10 Mbit/s. Therefore the intendeduser does not know that the transmission is not safe and cannot take anycountermeasures.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a solution so thatactive eavesdroppers interfering in wireless transmissions between atransmitter and a receiver can be detected. Another object of theinvention is to provide a simple solution to the above mentioned issue.

The above mentioned object are achieved by a method in a wirelesscommunication system for determining whether an active eavesdropper isinterfering in transmissions on a radio channel from a transmitter to areceiver, the method comprising the steps of:

-   -   a) estimating, by said transmitter, a first effective channel        gain of said radio channel at said receiver;    -   b) estimating, by said receiver, a second effective channel gain        of said radio channel at said receiver; and    -   c) determining whether an active eavesdropper is interfering in        the transmissions by comparing said first and second effective        channel gains.

Different preferred embodiments of the invention are defined in theappended dependent claims. The present method can also be executed inprocessing means and be comprised in a suitable code means.

According to another aspect of the invention the above mentioned objectis achieved by a method in a receiver for determining whether an activeeavesdropper is interfering in transmissions on a radio channel from atransmitter to said receiver, the method comprising the steps of:

-   -   d) receiving a first effective channel gain of said radio        channel at said receiver, said first effective channel gain        being estimated by said transmitter;    -   e) estimating a second effective channel gain of said radio        channel; and    -   f) determining whether an active eavesdropper is interfering in        the transmissions by comparing said first and second effective        channel gains.

The present method in a receiver can be implemented in a suitablereceiver device.

With the present invention the existence of an active eavesdropperinterfering in transmissions between a transmitter and a receiver can beefficiently detected. Further, the detection can be reported back to thetransmitter and the additional overhead can be held small and may onlybe related to the number of bits in the quantization.

Further applications and advantages of the invention will be apparentfrom the following detailed description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a data packet according to an embodiment of thepresent invention; and

FIG. 2 is a system overview and a process flow of an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

This present invention provides a simple, yet powerful, solution to aneavesdropper's attack. It should be noted that there is absolutelynothing discussed in prior art so far in the present topic. In fact, noteven the problem itself which the present invention solves has beenobserved for massive MIMO systems which also implies that no prior artsolutions exists.

To provide a deeper understanding of the present invention the followingbackground is provided: Assume that the wireless system using massiveMIMO operates in Time Division Duplex (TDD) mode, and the trainingsymbols are sent from the users since there are too many antennas at thetransmitter, e.g. a BS, to send orthogonal training symbols from thatside. Then, one relies on channel reciprocity and assumes that thedownlink channel=uplink channel (possibly compensated for the hardwareof the BS).

Let the channel vector from the IU to the BS be g_(IU)=√{square rootover (β_(IU))}h_(IU) where β_(IU) is a scalar measuring the shadowfading to the IU, and h_(IU) is a 1×M vector that models the small-scalefading, and M is the number of BS antennas (assumed large) in the MIMOtransmissions. The channel to the BS from the ED is similarly written asg_(ED)=√{square root over (β_(ED))}h_(ED).

The IU transmits a pilot symbol p to the BS, which receives r=p√{squareroot over (β_(IU))}h_(IU)+n. The receiver now estimates the channel fromthe IU as g_(IU)=r/p=√{square root over (β_(IU))}h_(IU)+n/p. Let usdenote the variance of n/p by N₀.

The IU constructs a beamforming vector by taking the complex transposeof the channel estimate

$f = {\frac{g_{IU}^{H}}{\sqrt{M}{g_{IU}^{H}}}.}$This gives an average transmit power of 1/M.

At the IU, the effective channel will be y=αx+w, where α=g_(IU)f.Asymptotically in M we have that

$\left. {\alpha }^{2}\rightarrow\frac{\beta_{IU}^{2}}{\beta_{IU} + N_{0}} \right.,\left. M\rightarrow{\infty.} \right.$The noise density at the IU is N_(IU) and is unknown to the BS.

At the same time, since h_(IU) and h_(ED) are independent, we have thatg_(ED)f→0, M→∞ which means that the ED will not receive any power atall, and hence the SC is perfect in the sense that it equals the Shannoncapacity, i.e.,

$\left. C_{SC}\rightarrow\left. C_{IU}\rightarrow{\log\left( {1 + \frac{\beta_{IU}^{2}}{N_{0}\left( {\beta_{IU} + N_{0}} \right)}} \right)} \right. \right.,\left. M\rightarrow\infty \right.$

We therefore conclude that there is no danger to system with a passiveED as the ED cannot overhear the transmission.

We now change the operations of the ED and the ED will be active andwill send pilot signals overlapping with the IU's pilot signals. This isa worst case assumption as, in reality, the ED will possibly be somewhatmismatched in time/frequency with the pilot signal sent from the IU.This will have the effect that some power will be beamformed to the EDinstead of to the IU. How much the ED will get of this power will dependon the relative powers of the pilot signals from the ED and the IU. Wehave the following situation:

-   -   The BS receives r=p√{square root over (β_(IU))}h_(IU)+√{square        root over (γ)}p√{square root over (β_(ED))}h_(ED)+n.    -   The BS does not know whether r contains the channel to the ED or        not, so the BS will estimate the channel to the IU as        g_(IU)=r/p=√{square root over (β_(IU))}h_(IU)+√{square root over        (γ)}√{square root over (β_(ED))}h_(ED)+n/p.    -   The IU constructs a beamforming vector by taking the complex        transpose of the channel estimate

$f = {\frac{g_{IU}^{H}}{\sqrt{M}{g_{IU}^{H}}}.}$This gives an average transmit power of 1/M.

-   -   The IU receives a signal y=α_(IU)x+w with

${\alpha_{IU}}^{2} = {\frac{\beta_{IU}^{2}}{\beta_{IU} + {\gamma\beta}_{ED} + N_{0}}.}$

-   -   The ED receives y=α_(ED)x+w with

${\alpha_{ED}}^{2} = {\frac{\gamma^{2}\beta_{ED}^{2}}{\beta_{IU} + {\gamma\beta}_{ED} + N_{0}}.}$

-   -   In this case, the SNR at the ED can be larger than at the IU        without that the IU or the BS knows about it. This may cause the        SC to be 0.

Hence, we must develop a technique that allows the system todetermine/detect whether there is an active ED present or notinterfering in the transmissions. This is achieved by the presentinvention. The transmitter estimates a first effective channel gain ofthe radio channel at the receiver, i.e. the IU. The receiver (IU)estimates a second effective channel gain of the radio channel at thereceiver. And finally it is determined whether an active ED isinterfering in the transmissions from the transmitter to the receiver bycomparing the first and second effective channel gains.

It is assumed that the active ED transmits one or more pilot signals tointerfere in the mentioned transmissions. Further, the square of thefirst and second effective channel gains are proportional to the SNR atthe receiver according to an embodiment of the invention.

The step of determining may be performed in any suitable node of thesystem, e.g. a central control node. The determining step may preferablycomprise calculating a ratio of the first and second effective channelgains; and thereafter comparing the ratio with a threshold value so asto determine whether an active ED is interfering in the transmissions ornot. It has been concluded that the ratio may be compared to a thresholdvalue, and it is determined that an ED is present if the ratio is equalto or less than the threshold value. It should however be noted that acomparison of the difference between the first and second effectivechannel gains may be performed to determine if an active ED isinterfering in the transmissions, e.g. Log-values of the first andsecond effective channel gains, etc.

The threshold value is preferably simulated and predefined, and based onthe number of transmit antennas in the MIMO transmission from thetransmitter, so to determine the threshold, simulation must beperformed. These simulations will be specific to the number of transmitantennas M. The more antennas, the more accurate will the estimations beand the higher value for the threshold value can take.

The following disclosure gives a more detailed explanation of thedifferent steps of an embodiment of the present invention.

Step 1: Channel Estimation

The transmitter forms a channel estimate g_(IU)=r/p where r is thereceived signal at the IU's pilot slot and p is the pilot symbol. Thenoise per element is N₀ which is for notational convenience and impliesthat the AWGN variance is p²N₀ We assume that the value of N₀ is knownat the transmitter which is a mild constraint as this is long-termconstant and can be estimated through standard methods. How to estimateN₀ is not a part of the present invention. Further, the noise densitycan be measured outside the transmission bandwidth so that a highquality estimate can be obtained.

Step 2: IU Effective Channel Prediction

In the case that the ED is not present, the effective channel gain atthe IU will have square magnitude

${\alpha }^{2} \approx {\frac{\beta_{IU}^{2}}{\beta_{IU} + N_{0}}.}$In order to estimate this value, we must estimate β_(IU). We can do thisas we know that

${\frac{g_{IU}g_{IU}^{H}}{M}->{\beta_{IU} + N_{0}}},{M->{\infty.}}$Therefore, we can compute an estimate of |α|² as,

${\beta_{IU} \approx {\frac{g_{IU}g_{IU}^{H}}{M} - N_{0}}},{\left\lbrack {\alpha }^{2} \right\rbrack_{EST} = {\frac{\beta_{IU}^{2}}{\beta_{IU} + N_{0}} = {\frac{\left( {\frac{g_{IU}g_{IU}^{H}}{M} - N_{0}} \right)^{2}}{\frac{g_{IU}g_{IU}^{H}}{M}}.}}}$

This estimated value will now be quantized and can added to the datapacket in FIG. 1.

Step 3: Channel Estimation at the IU

This step needs to be done no matter if there is PLS or not. However,for the present application, the problem is simpler than the channelestimation problem as we do not need any phase information. Given thatthe noise density is known at the IU—and, again, noise is long termconstant and can be estimated with a variety of methods (it needs to bedone anyway in most systems)—then we can estimate the power of thechannel gain as,

$\left\lbrack {\alpha }^{2} \right\rbrack_{{IU} - {EST}} = {{\frac{1}{N}{\sum\limits_{n = 1}^{N}{y_{k}}^{2}}} - {N_{0,{{IU} - {EST}}}.}}$

This is because we have the model y=αx+w, so that the expected power persample in y becomes |α|²+N_(0,IU) (under the standard assumption of unitaverage energy symbols). Such estimation requires that the observationwindow is fairly large. In cases where the observation window is short,a dedicated training symbol can be inserted. In this case, the firstsymbol in v would be known, and we can assume it to be unity. Then, wecan estimate the power of the channel gain according to theMMSE-criteria as,

$\left\lbrack {\alpha }^{2} \right\rbrack_{{IU} - {EST}} = {\frac{{y_{1}}^{2}}{1 + N_{0,{IU}}}.}$Step 4 and 5: Comparison with Threshold Value and Reporting

The IU now computes the ratio

$\varphi = \frac{\left\lbrack {\alpha }^{2} \right\rbrack_{{IU} - {EST}}}{\left\lbrack {\alpha }^{2} \right\rbrack_{EST}}$(or a difference value) and compares the ratio with a threshold value δ.If φ<δ, then the IU declares an attack and reports this to thetransmitter via a feedback link, and the transmitter stops thetransmissions in response to message from the IU. Otherwise, there is noattack declared and the transmission continues.

Hence, according to an embodiment of the invention, the step ofdetermining is performed in the receiver (IU). In this respect a newprotocol between the transmitter and receiver according to the inventionis presented which also was mentioned in the above example. The protocolwill allow the IU to identify whether there is an ED attacking thesystem or not. If so, the IU will inform the transmitter to stop thetransmissions. FIG. 1 gives an example of a data packet according to theinvention. The data packet transmitted from the transmitter to the IUmay have the form as shown in FIG. 1. The only additional overheadneeded is a quantized version of the estimated effective channel at theIU.

In summary, with reference to FIG. 1, the following operations need tobe done according to this embodiment:

-   -   The transmitter (e.g. a BS) estimates the channel by predicting        the power of the effective channel (the first effective channel        gain) at the IU and encodes this information into a payload data        packet.    -   The IU performs channel estimation at the IU and computes the        power of the second effective channel gain. Note that this step        is required in all systems, so that this does not bring any        additional overhead to the system.    -   The IU compares the second estimated channel gain with the        predicted first effective channel gain received from the        transmitter.    -   If the ratio of the two gains is smaller than a pre-defined        threshold, the IU declares an attack and requests the        transmitter to stop the transmission via a feedback link.

FIG. 2 illustrates a system overview of an embodiment of the presentinvention. The system is in this example a cellular wirelesscommunication system, such as LTE or GSM. The transmissions areperformed in the downlink and the uplink of the system. It shouldhowever be noted the present invention is a general method for detectingactive EDs and is not limited to a particular wireless communicationsystem. With reference to FIG. 2:

-   -   In step A, the base station transmits a first effective channel        gain to the mobile station.    -   In step B, the mobile station compares the first effective        channel gain with the second effective channel gain and        determines if an ED is interrupting in the transmissions between        the base station and the mobile stations.    -   In step C, the mobile station request a stop of transmissions if        it was determined in step 2 that an ED was interrupting in the        transmissions.

Furthermore, as understood by the person skilled in the art, any methodaccording to the present invention may also be implemented in a computerprogram, having code means, which when run by processing means causesthe processing means to execute the steps of the method. The computerprogram is included in a computer readable medium of a computer programproduct. The computer readable medium may comprises of essentially anymemory, such as a ROM (Read-Only Memory), a PROM (Programmable Read-OnlyMemory), an EPROM (Erasable PROM), a Flash memory, an EEPROM(Electrically Erasable PROM), or a hard disk drive.

The present method for determining whether an active eavesdropper isinterfering in transmissions on a radio channel from a transmitter to areceiver will in the receiver comprise the steps of: receiving a firsteffective channel gain of the radio channel at the receiver, the firsteffective channel gain being estimated by the transmitter; estimating asecond effective channel gain of the radio channel; and determiningwhether an active eavesdropper is interfering in the transmissions bycomparing the first and second effective channel gains.

The invention also relates to a receiver device comprising the suitablemeans, elements, units, and being arranged to execute the above method,Mentioned means, units, elements, may e.g., be memory, processingcircuitry, coupling means, antenna means, precoding unit, amplifierunit, etc. The present receiver may e.g. be a UE in the LTE system. Thereceiver may receive the first effective channel gain from a transmitterdevice of the system.

The present receiver device for determining whether an activeeavesdropper is interfering in transmissions on a radio channel from atransmitter to said receiver, the device comprises: a receiving unitarranged for receiving a first effective channel gain of a radio channelat said receiver, the first effective channel gain being estimated bythe transmitter; an estimating unit arranged to estimating a secondeffective channel gain of the radio channel; and a determining unitarranged to determining whether an active eavesdropper is interfering inthe transmissions by comparing the first and second effective channelgains.

A receiver, also known as UE in LTE systems, mobile station, wirelessterminal and/or mobile terminal is enabled to communicate wirelessly ina cellular wireless communication system. The receiver may further bereferred to as mobile telephones, cellular telephones, computer tabletsor laptops with wireless capability. The receivers in the presentcontext may be, for example, portable, pocket-storable, hand-held,computer-comprised, or vehicle-mounted mobile devices, enabled tocommunicate voice and/or data, via the radio access network, withanother entity.

In some radio access networks, several transmitters may be connected,e.g., by landlines or microwave, to a Radio Network Controller (RNC),e.g., in Universal Mobile Telecommunications System (UMTS). The RNC,also sometimes termed Base Station Controller (BSC), e.g., in GSM, maysupervise and coordinate various activities of the plural transmittersconnected thereto. In 3rd Generation Partnership Project (3GPP) LongTerm Evolution (LTE), transmitters, which may be referred to as eNodeBsor eNBs, may be connected to a gateway, e.g., a radio access gateway, toone or more core networks.

Finally, it should be understood that the present invention is notlimited to the embodiments described above, but also relates to andincorporates all embodiments within the scope of the appendedindependent claims.

What is claimed is:
 1. A method in a wireless communication system fordetermining whether an active eavesdropper interferes in transmissionson a radio channel from a transmitter to a receiver, the methodcomprising: estimating, by the transmitter, a first effective channelgain of the radio channel at the receiver; estimating, by the receiver,a second effective channel gain of the radio channel at the receiver;obtaining, a ratio or a difference value of the first and secondeffective channel gains; determining whether the active eavesdropperinterferes in the transmissions by comparing the ratio or differencevalue with a threshold value; and wherein the threshold value issimulated based on a quantity of transmit antennas in Multi-inputMulti-output (MIMO) transmissions.
 2. The method according to claim 1,wherein the determining is performed by the receiver.
 3. The methodaccording to claim 1, wherein the active eavesdropper interferes in thetransmissions, if the ratio or difference value is less than or equal tothe threshold value.
 4. The method according to claim 1, wherein thetransmitter employs Multi-input Multi-output (MIMO) transmissions to thereceiver.
 5. The method according to claim 1, wherein a representationof the first effective channel gain is encoded in a data packettransmitted to the receiver.
 6. The method according to claim 5, whereinthe data packet is transmitted by the transmitter.
 7. The methodaccording to claim 1, wherein the active eavesdropper transmits pilotsignals so as to interfere in the transmissions.
 8. The method accordingto claim 1, further comprising: signaling a request to stop thetransmissions based on whether the active eavesdropper interferes in thetransmissions.
 9. The method according to claim 8, wherein thetransmitter stops the transmissions when receiving the request.
 10. Themethod according to claim 9, wherein the request is signaled by thereceiver to the transmitter.
 11. The method according to claim 1,wherein the transmitter is a base station and the receiver is a mobilestation in a cellular communication system.
 12. The method according toclaim 1, wherein a first square value of the first effective channelgain and a second square value of the second effective channel gain areproportional to the Signal-to-Noise Ratio (SNR) at the receiver.
 13. Amethod for determining whether an active eavesdropper interferes intransmissions on a radio channel from a transmitter to a receiver, themethod comprising: receiving, by the receiver, a first effective channelgain of the radio channel at the receiver, the first effective channelgain being estimated by the transmitter; estimating, by the receiver, asecond effective channel gain of the radio channel at the receiver;obtaining, a ratio or a difference value of the first and secondeffective channel gains; determining whether the active eavesdropperinterferes in the transmissions by comparing the ratio or differencevalue with a threshold value; and wherein the threshold value issimulated based on a quantity of transmit antennas in Multi-inputMulti-output (MIMO) transmissions.
 14. The method according to claim 13,wherein the transmitter is a base station and the receiver is a mobilestation in a cellular communication system.
 15. The method according toclaim 13, wherein the active eavesdropper interferes in thetransmissions, if the ratio or difference value is less than or equal tothe threshold value.
 16. A receiver device arranged for communication ina wireless communication system and further arranged for determiningwhether an active eavesdropper interferes in transmissions on a radiochannel from a transmitter to the receiver; the receiver devicecomprising: a receiver, configured to receive a first effective channelgain of a channel at the receiver device, the first effective channelgain being estimated by the transmitter; an estimator, configured toestimate a second effective channel gain of the radio channel; aprocessor, configured to: obtain, a ratio or a difference value of thefirst and second effective channel gains; determine whether the activeeavesdropper interferes in the transmissions by comparing the ratio ordifference value with a threshold value; and wherein the threshold valueis simulated based on a quantity of transmit antennas in Multi-inputMulti-output (MIMO) transmissions.
 17. The receiver according to claim16 wherein the transmitter is a base station and the receiver is amobile station in a cellular communication system.
 18. The receiveraccording to claim 16, wherein the active eavesdropper interferes in thetransmissions, if the ratio or difference value is less than or equal tothe threshold value.
 19. A non-transitory computer readable mediumhaving computer executable instructions which when executed by acomputer processor causes the computer processor to execute thefollowing: receiving a first effective channel gain of a radio channelat a receiver device, the first effective channel gain being estimatedby a transmitter; estimating, at the receiver device, a second effectivechannel gain of the radio channel; and obtaining, a ratio or adifference value of the first and second effective channel gains;determining whether an active eavesdropper interferes in thetransmissions by comparing the ratio or difference value with athreshold value; and wherein the threshold value is simulated based on aquantity of transmit antennas in Multi-input Multi-output (MIMO)transmissions.
 20. The non-transitory computer readable medium accordingto claim 19 wherein the transmitter is a base station and the receiveris a mobile station in a cellular communication system.
 21. Thenon-transitory computer readable medium according to claim 19, whereinthe active eavesdropper interferes in the transmissions, if the ratio ordifference value is less than or equal to the threshold value.